The goal of this project is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. Toxicity testing is an important aspect of public health research in that it serves to identify chemicals that are hazardous to human health. Proper conduct of toxicology studies is required to ensure that the resulting data are reliable. This project involves the characterization of the acute toxicity of trimethylsilyldiazomethane (TMSD). TMSD is a methylating agent used in synthetic chemistry. It is marketed as a safe replacement for diazomethane, primarily due to a lower explosive potential. However, two chemists died of acute pulmonary edema while working with TMSD in the laboratory. NTP is performing toxicity studies in order to further examine the potential for TMSD to induce pulmonary toxicity. The data from this project will be used in the development of sound, scientific conclusions about the potential toxicity and carcinogenicity of TMSD in rats and mice and ultimately supports the risk assessment efforts of National Toxicology Program and other federal agencies. In addition, the public can be adequately informed about risk factors arising from exposure to TMSD. Keywords: toxicity, inhalation, trimethylsilyldiazomethane, lung